Will transformer oil or engine oil dripping onto optical fiber damage the glass by permeating into it?
It will not damage or permeate the silica glass body, but may cause physical or chemical damage to the polymer coating on the outer layer of the optical fiber.
Here is a detailed analysis from the perspectives of material physics and chemistry:
I. Impact of Oil Stains on the Glass Body (Core and Cladding)
- Chemical Inertness:
The core and cladding of optical fibers are mainly composed of high-purity silica (\text{SiO}_2). Silica exhibits a highly dense covalent inorganic network structure with excellent chemical stability. Transformer oil (primarily insulating mineral oil composed of alkanes, cycloalkanes, and aromatic hydrocarbons, or synthetic esters) and machine oil (hydrocarbon base oil with additives) are both non-polar organic substances. At normal temperatures and most industrial operating temperatures, they will not react chemically with silica. - Molecular Permeability:
Large oil molecules cannot penetrate the defect-free, dense silica glass network. Therefore, when oil droplets come into contact with the bare glass surface of the optical fiber, they will not diffuse within the glass or alter the refractive index distribution of the glass body.
II. Impact of Oil Stains on the Optical Fiber Coating and Potential Risks
Although the glass body is oil-resistant, the coating material on the outermost layer of the optical fiber determines its actual lifespan in an oily environment:
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Standard Acrylate Coating (Ordinary Communication Optical Fiber):
- Ordinary optical fibers (typically operating at temperatures from -40\ ^{\circ}\text{C} \sim +85\ ^{\circ}\text{C}) use UV-curable acrylate coatings.
- Swelling and Aging: When long-term immersed in machine oil or hot transformer oil (especially at temperatures exceeding 70\ ^{\circ}\text{C} \sim 80\ ^{\circ}\text{C}), organic small molecules may penetrate the acrylate polymer, causing the coating to swell, soften, or even detach.
- Loss of Mechanical Strength: Once the coating fails, the internal 125\ \mu\text{m} quartz glass loses its buffering and scratch protection. The surface is prone to micro-cracks and brittle fracture under slight bending or stress.
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Optical Additional Loss (if oil contaminates the optical path):
- The refractive index of transformer oil and machine oil is typically around n \approx 1.45 \sim 1.50, which is close to that of the silica glass cladding (approximately 1.444). If the end face of an optical fiber connector is contaminated with oil, it will significantly alter the reflection and coupling characteristics of the optical end face, leading to a sharp increase in optical loss or deterioration of return loss.
III. Selection Recommendations for Oil-Immersed Industrial Environments
In harsh environments such as temperature measurement inside oil-immersed transformers or engine oil monitoring, special chemically resistant optical fibers and metal protective sleeving solutions are typically employed:
- Polyimide (PI) Coated Optical Fiber:
Polyimide possesses excellent thermal resistance and chemical solvent resistance, enabling it to withstand long-term immersion in transformer insulating oil, various lubricating oils, and fuel oils.
Related Special Optical Fiber Products:
- OFSCN® 300℃ SM Polyimide Optical Fiber
Utilizes a polyimide coating, with an operating temperature range of -200\ ^{\circ}\text{C} \sim +350\ ^{\circ}\text{C}, offering excellent resistance to chemical media corrosion and high temperatures.
- Fiber in Metal Sheathing (FIMT / Seamless Steel Tube):
In engineering applications with heavy oil contamination, high pressure, or mechanical shock, seamless stainless steel tubes are often used to hermetically seal and protect the optical fiber, completely isolating it from oil media and mechanical stress.